Lithium a-Ethoxycarbonylvinyl(1-hexynyl)cuprate

[54111-85-2]  · C11H16CuLiO2  · Lithium a-Ethoxycarbonylvinyl(1-hexynyl)cuprate  · (MW 250.76)

(organometallic reagent for the introduction of an acrylate unit onto allylic halides, vinyl epoxides, and carbonyl compounds; also effective for the synthesis of a-methylene-g-butyrolactones from allylic halides)

Physical Data: thermally unstable organocuprate generated at -78 °C in the absence of moisture and oxygen.

Solubility: sol diethyl ether and THF at temperatures approaching -78 °C.

Preparative Methods: generated at -78 °C in ether by the metal-halogen exchange reaction of ethyl a-bromoacrylate and lithium 1-hexynyl(methyl)cuprate.1

Handling, Storage, and Precautions: must be prepared in the absence of moisture and oxygen at -78 °C. Diethyl ether should be freshly distilled from lithium aluminum hydride. Reaction temperatures should be maintained below -60 °C.

Generation and Reactivity of a-Alkoxycarbonylvinylcuprates.

In general, a-alkoxycarbonylvinylic cuprates have been generated in situ from the conjugate additions of organocopper reagents to alkynic esters and acids.2-4 The unsubstituted title reagent can best be prepared by the reaction of methylcuprates with ethyl a-bromoacrylate (eq 1).1 This metal-halogen exchange was first observed by Klein and Levene for substituted a-alkoxycarbonylvinylcuprates.5 While oxidation, protonation, and iodination of the vinylcuprates have met with success, alkylations with simple alkyl halides other than methyl iodide have not been reported.4

The parent a-ethoxycarbonylvinylcuprate selectively reacts with allyl halides and propargyl halides to produce 1,3-dienes (2) and 1,3-enynes (eq 2).1 The adduct (3) of the title reagent with 3-bromocyclohexene produces the acrylic acid (4) after basic hydrolysis (eq 3). Iodolactonization of acid (4) and subsequent deiodination with Tri-n-butylstannane yields the a-methylene-g-butyrolactone (6) (eq 4).1

An application of cuprate (1) in the synthesis of swazinecic acid dilactone6 is provided in eq 5 for the allylation of halide (7).

Reactions with Carbonyl Compounds.

By analogy to reagent (1), a-methoxycarbonylvinylcuprate (8) was prepared from methyl a-bromoacrylate according to the aforementioned procedure.1 A study of the reactivity of reagent (8) with ketones and a,b-unsaturated ketones was carried out. Unlike most organocuprates, reagent (8) added in a 1,2-manner to the carbonyl group of a series of cyclic ketones at -78 °C in high yield (eq 6).7

More unusual is the fact that cuprate (8) preferentially adds 1,2 and not 1,4 to a variety of enones.7 Cyclohexenone, cyclohexenecarbaldehyde, and acylcyclohexenes all gave 1,2-adducts exclusively with cuprate (8) (eqs 7 and 8). Only the reaction of (8) with methyl vinyl ketone resulted in the 1,4-adduct being the major product (eq 9).

This reactivity profile suggested that the a-alkoxycarbonylvinylcuprates are structurally indicative of an allenoate species. Evidence for the allenoate-type structure (9) comes from IR data5 and trapping of substituted a-alkoxycarbonylvinylcuprates generated in the conjugate addition of alkylcuprates to ethyl propiolate (eq 10).8 After the addition of Lithium Dimethylcuprate to ethyl propiolate at -78 °C, the reaction mixture was quenched with Chlorotrimethylsilane in triethylamine. The O-silylated allene (10) was isolated and characterized by IR and NMR.

1. Marino, J. P.; Floyd, D. M. JACS 1974, 96, 7138.
2. Corey, E. J.; Katzenellenbogen, J. A. JACS 1969, 91, 1851.
3. Klein, J.; Levene, R. JCS(P2) 1973, 1971.
4. Siddall, J. B.; Biskup, M.; Fried, J. H. JACS 1969, 91, 1853.
5. Klein, J.; Levene, R. JACS 1972, 94, 2520.
6. Gordon-Gray, C. G.; Whiteley, C. G. JCS(P1) 1977, 2040.
7. Marino, J. P.; Floyd, D. M. TL 1975, 3897.
8. Marino, J. P.; Linderman, R. J. JOC 1981, 46, 3696.

Joseph P. Marino & David P. Holub

University of Michigan, Ann Arbor, MI, USA

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